There is known a method for forming a metal film by preparing a liquid or paste ink of a metal, and applying or printing the ink on a substrate, followed by warming. The metal used here may be gold, silver, copper, or aluminum, with silver being generally used as a wiring material. Ink with silver, which usually contains silver metal dispersed in a dispersion solvent, is patterned on an interconnection substrate, and silver metal in the ink is sintered to form wiring. It is known that, when used as an electrically conductive material, silver metal is required to be sintered at a lower temperature, making use of melting point depression caused by the fine size of the dispersed silver metal particles. Fine metal nanoparticles of a nanoscale are raising expectations as a material sintering at a low temperature.
On the other hand, silver metal particles of such a fine size as to cause melting point depression are prone to contact and aggregate with each other, and a dispersant is required in the ink for preventing such aggregation. However, sintering the silver metal particles in the ink containing a dispersant may result in residual impurities originated from the dispersant. It is thus usually desired to remove the impurities by a high-temperature treatment or the like.
A general method of preparing silver nanoparticles includes, for example, reducing a silver salt composed of an inorganic acid and silver, such as silver nitrate, in the presence of a dispersant. With this method, however, a residual acid component originated from the silver salt may result, and high-temperature treatment is required for removal of the dispersant.
A method of forming silver metal with a silver salt containing an organic acid, instead of the inorganic acid, has also been reported. The organic silver proposed is, for example, silver salt of long-chain carboxylic acid, which is thermally decomposed or reduced in the presence of an alkylamine (Patent Publications 1 to 5). To the silver nanoparticles prepared from any organic silver, carboxylic acid or silver carboxylate is adhered, which is nonvolatile and thermally decomposed at higher temperatures. Thus in order to obtain a silver element, such as silver film or line, of excel lent electrical conductivity, heat-treatment at not lower than 200° C. for a prolonged period of time may be required.
There have recently been made active attempts to form silver metal on a transparent resin substrate. A transparent resin substrate generally has a lower softening point compared to glass or the like, and thus a silver-forming material sintering at low temperatures is desired which realizes formation of silver metal through heating at not higher than 150° C. Sintering at low temperatures calls for removal at low temperatures of the dispersant adhered on the silver nanoparticles. Further, for the production of a durable metal-resin substrate, strong bond is required between the transparent resin substrate and the silver element.
As means for low temperature sintering, there has been proposed a method of mixing silver oxalate and amine to produce silver nanoparticles of silver oxalate-amine complex having a thermal decomposition temperature of 110° C. (Patent Publication 6). The silver nanoparticles obtained by this method provide a silver element, such as a metal film having good electrical conductivity, through sintering at not higher than 150° C., but this publication discloses nothing about durability, including adhesivity, of the obtained silver element. It is assumed that pure silver element obtained by such a mechanism of action to leave no residual organic substance in the resulting film has poor adhesivity to a substrate.
On the other hand, as means for improving adhesivity or strength of a silver element, there has been reported to use a composition wherein an organic polymer having a urethane group and a diol group is hybridized with an inorganic material (Patent Publication 7). However, with an organic polymer, which will remain as a resistive component in the resulting electrically conductive film, electrical conductivity comparable to bulk silver is hard to be achieved.
Patent Publication 1: JP 2005-60824-A
Patent Publication 2: JP 2007-84879-A
Patent Publication 3: WO 2004/012884
Patent Publication 4: JP 2004-27347-A
Patent Publication 5: JP 2009-185390-A
Patent Publication 6: JP 2010-265543-A
Patent Publication 7: WO 2007/148684